@ Bajaman,I've been doing some simulations on this circuit and you are spot on about the ZTX951 pulling down the frequency response above 1khZ while most other BJT's don't.

But interesting that all the transistors (available in my sim) including the ZTX951 do seem to improve the waveform (more rounded) when compared to just using 1N914 or similar. Diodes show harder edges on the output waveform, especially when the signal is large.

My point being; Maybe any BJT will at least change the waveform and the addition of that larger 470pF mentioned earlier to help osculations might still sound good, better or just plain old different.

For those that don't have the XTX951 you might find it useful to try other transistors.Of course I've not built this and sims don't always tell it like it really is so suck it and see. Phil.

mwelch55 wrote:I was wondering why Friedman and others have Vbias going through an op amp? Is there an advantage to this like less noise or is this just something you do with unused op amps?

Both.

It puts to good use a spare opamp and it supplies a bias voltage at much lower source impedance (aka as a stiffer bias voltage) than using 2 resistors. Ok the decoupling capacitor reduces the source impedance (of a simple potential divider) at higher frequencies but as you approach DC the source impedance is equal to the 2 voltage divider resistors in parallel, so if you used 100k then the source impedance near DC would be 50k.

With an opamp the source impedance will be somewhere between 100 ohms and 1k (depends upon which opamp), this will reduce possible interaction between stages due to bias voltage fluctuations.

You may ask does it matter that the bias voltage is low impedance or not. Well sometimes it does, sometimes it doesn't but IMO it is good practice and is one of several things you can do at the design stage to make a circuit that will behave as well as possible under differing environments.

/rantEvery time I read of a pedal that squeals at certain settings, or picks up radio signals, or clicks, or exhibits other undesirable traits that indicates to me that the "designer" has not taken due care to eliminate potential problems by using good design techniques, and/or has not tested it under real life situations and/or doesn't care.rant/

Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

My pedal is sounding a bit dark. As in I feel the treble and pres needs to be on max to sound normal.All my component values check out. Is anyone else finding this?There are a lot of caps to ground, I guess I'll start reducing them and see what happens...

mwelch55 wrote:I was wondering why Friedman and others have Vbias going through an op amp? Is there an advantage to this like less noise or is this just something you do with unused op amps?

screenshot.642.png

This is a more stable bias point than a capacitively decoupled resistive divider. Output impedance is actually Zero, as it is an "active ground" and achieves zero output impedance due to negative feedback. It is limited by the maximum output capability of the op-amp however.

I wrote this earlier in the thread, but it is worth noting again. Not only is a resistive divider high impedance, but no amount of capacitance will make it a stable bias point if you are creating assymetrical signals. More capacitance will just create longer and longer time constants before stability is achieved. This won't always be a problem, but bias shifts usually don't sound good, especially with multiple DC coupled stages. Worth knowing, as it may help you debug a circuit someday.

DC gain and DC coupling has much more current capability than a capacitively filtered resistive divider, so It won't be a very predictable reference voltage to rely on R's and C's. Hope that is helpful.

bmxguitarsbmx wrote: Output impedance is actually Zero, as it is an "active ground" and achieves zero output impedance due to negative feedback. It is limited by the maximum output capability of the op-amp however.

+1

Ignore my figures above, it's a while since I did any analogue design and I forgot about the effect of feedback!

Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

Finally got some time to complete my build - I call it "pink eye" It fired from the first try without any issues. I put NE5332P for U1 and U2 because I had those in my bin (shouldn't be any worse than TL072) and TL073 for U3. Other than that, it's a stock circuit, but I put sockets to be able to play with all clipping diodes and with the 120pF capacitor. I'll play with this version for a few days and then I'll experiment with ZTX clippers.

Didn't have time to thoroughly test it this morning, but I got to plug it into the mini Blackstar Fly3 and it sounds promising. It's got that raw hot-rodded Marshall vibe. Knobs do less than I'd expect in a way that they don't really change the character of the pedal, but they do tweak the sound nicely.

Will try to make few recordings over the weekend and follow up with the clips.

bancika wrote:Finally got some time to complete my build - I call it "pink eye" It fired from the first try without any issues. I put NE5332P for U1 and U2 because I had those in my bin (shouldn't be any worse than TL072) and TL073 for U3. Other than that, it's a stock circuit, but I put sockets to be able to play with all clipping diodes and with the 120pF capacitor. I'll play with this version for a few days and then I'll experiment with ZTX clippers.

Didn't have time to thoroughly test it this morning, but I got to plug it into the mini Blackstar Fly3 and it sounds promising. It's got that raw hot-rodded Marshall vibe. Knobs do less than I'd expect in a way that they don't really change the character of the pedal, but they do tweak the sound nicely.

Will try to make few recordings over the weekend and follow up with the clips.

@andlord yes, those are 1/4w resistors. I can't find 1/8w where I live so these were the smallest available.

The switch is from musikding.de, they have few colors. One of my favorite shops

@picatostas Haven't compared shielded vs regular, but I always use shielded in high gain pedals and amps. As I was testing the pedal all finished except for jack wiring, I used alligator clips to connect inputs and outputs to guitar/amp and it was squealing and humming like crazy, so I thought that it would be beneficial to shield the leads.

bancika wrote:@andlord yes, those are 1/4w resistors. I can't find 1/8w where I live so these were the smallest available.

The switch is from musikding.de, they have few colors. One of my favorite shops

@picatostas Haven't compared shielded vs regular, but I always use shielded in high gain pedals and amps. As I was testing the pedal all finished except for jack wiring, I used alligator clips to connect inputs and outputs to guitar/amp and it was squealing and humming like crazy, so I thought that it would be beneficial to shield the leads.

As the jacks are grounds, so the pots , you practically reduce all that noise considerably